Finned tube for heat exchangers, heat exchanger, process for producing heat exchanger finned tube, and process for fabricating heat exchanger

ABSTRACT

A heat exchanger finned tube  10  for use in fabricating a heat exchanger  1  useful as the evaporator for refrigerators or the like wherein a hydrocarbon refrigerant is used. Two tube insertion holes spaced apart from each other are formed in each of plate fins  12 , and two straight tube portions  11   a  of a hairpin tube  11  are inserted through the respective holes of each plate fin to arrange the plate fins  12  in parallel into a plurality of fin groups 13 spaced apart on the straight tube portions  11   a  longitudinally thereof. The hairpin tube  11  is enlarged with use of a fluid to fixedly fit the plate fins  12  of each tin group  13  around an enlarged tube portion  14  of the hairpin tube  11  and provide a finless part  19  between each pair of adjacent fin groups  13  on each of the straight tube portions  11   a . A restrained small-diameter portion  15  is provided in each of the finless parts  19  of each straight tube portion  11   a . The heat exchanger  1  fabricated using the finned tube  10  exhibits the desired refrigeration performance with the leakage of refrigerant diminished.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is an application filed under 35 U.S.C. §111(a)claiming the benefit pursuant to 35 U.S.C. §119(e)(1) of the filing dateof Provisional Application No. 60/331,210 filed Nov. 13, 2001 pursuantto 35 U.S.C. §111(b).

TECHNICAL FIELD

[0002] The present invention relates to heat exchanger finned tubes foruse in fabricating heat exchangers useful as evaporators ofrefrigeration devices such as refrigerators and refrigerated showcases,heat exchangers, a process for producing the finned tube and a processfor fabricating the heat exchanger.

BACKGROUND ART

[0003] As evaporators of refrigeration devices such as refrigerators andrefrigerated showcases, heat exchangers are in use which comprise a heatexchanger finned tube and formed in a zigzag shape in its entirety bybending the finned tube at a plurality of finless portions thereof. Thefinned tube comprises a hairpin tube, and a plurality of fin groupswhich are arranged on two straight tube portions of the hairpin tubelongitudinally thereof at a spacing and each of which comprises aplurality of parallel plate fins extending across and fixed to the twostraight tube portions.

[0004] Such heat exchangers have heretofore been fabricated by thefollowing two processes.

[0005] The first of the processes is as follows. First prepared are twostraight tubes, a multiplicity of plate fins each having two holes, anda tube enlarging device comprising a wire and a tube enlarging ballattached to one end of the wire. The two tubes are then inserted throughthe respective holes of each plate fin to thereby arrange the plate finsin parallel into a plurality of fin groups as spaced apart on the tubeslongitudinally thereof. The wire of the tube enlarging device issubsequently inserted at the other end thereof through each tube andpulled at the other end to force the ball through the tube to enlargethe tube and fixedly fit the plate fins of each fin group around thetube. The two tubes are then welded, each at one end thereof, toopposite ends of a U-shaped bend to thereby interconnect the two tubesby the bend, whereby a heat exchanger finned tube is produced. Thefinned tube is thereafter bent into a zigzag form in its entirety atportions thereof having no fin groups. In this way, a heat exchanger isfabricated.

[0006] Studies are recently under way for the use of hydrocarbonrefrigerants which are less likely to destroy the ozone layer and toinfluence global warming, in refrigerators, refrigerated showcases andlike refrigeration devices as substitutes for chlorofluorocarbonrefrigerants. Since the hydrocarbon refrigerants are flammable, there isa need to diminish the leakage of the refrigerant.

[0007] The heat exchanger fabricated by the first conventional processnevertheless has the following problems. Since the finned tube has seamsbetween the U-shaped bend and the component tubes welded thereto, therefrigerant is likely to leak from the seam portions. Further in thecase where the finned tube has, for example, inner fins arranged on itsinner surface circumferentially thereof at a spacing and extendinglongitudinally of the tube so as to give an increased heat transfer areato the tube for an improved refrigeration efficiency, the enlarging ballis more likely to collapse the inner fins to result in increasedresistance to the flow of refrigerant and impaired refrigerationperformance if the inner fins have an excessive height. Accordingly, theinner fins can not be given an increased height and are less effectivefor an increase in the heat transfer area, hence a limitation to theimprovement in refrigeration efficiency.

[0008] Heat exchangers for use as evaporators in refrigerators andrefrigerated showcases are fabricated by the second process to bedescribed below. First prepared are a hairpin tube, a multiplicity ofplate fins each having two holes which are spaced apart, and a tubeenlarging device comprising a pressure rod and an enlarging mandrelattached to one end of the rod. The two straight tube portions of thehairpin tube are then inserted through the respective holes of eachplate fin to thereby arrange the plate fins in parallel into a pluralityof fin groups as spaced apart on the tube portions longitudinallythereof. The mandrel of the tube enlarging device is subsequently forcedinto the straight tube portions from each open end of the hairpin tubeto enlarge the tube portions and to fixedly fit the plate fins of eachfin group around the tube portions of the hairpin tube, whereby a heatexchanger finned tube is produced. The finned tube is thereafter bentinto a zigzag form in its entirety at portions thereof having no fingroups. In this way, a heat exchanger is fabricated.

[0009] The heat exchanger produced by the second process has no seams inthe hairpin tube of the finned tube, so that no leakage of therefrigerant occurs unlike the heat exchanger obtained by the firstprocess. However, the heat exchanger produced by the second process alsohas the following problem. In the case where the finned tube has, forexample, inner fins arranged on its inner surface circumferentiallythereof at a spacing and extending longitudinally of the tube so as togive an increased heat transfer area to the tube for an improvedrefrigeration efficiency, the enlarging mandrel is more likely tocollapse the inner fins to result in increased resistance to the flow ofrefrigerant and impaired refrigeration performance if the inner finshave an excessive height. Accordingly, the inner fins can not be givenan increased height and are less effective for an increase in the heattransfer area, hence a limitation to the improvement in refrigerationefficiency.

[0010] In order to prevent the inner fins from collapsing, therefore, itis thought useful to enlarge the hairpin tube in its entirety byintroducing a pressure fluid into the tube in the second process. Inthis case, however, the circumferential wall of the tube wrinkles inportions thereof having no fin groups, deforming the tube longitudinallythereof to vary the length of the finned tube and failing to afford aheat exchanger of desired dimensions. The heat exchanger finned tube isbent at a plurality of finless portions thereof, whereas the tube islikely to collapse when bent if wrinkles or creases develop in suchportions. Further before the hairpin tube is enlarged, the straight tubeportions of the tube are not restrained in any way of course at thefinless parts thereof or at the portions thereof provided with the fingroup, so that the application of the pressure fluid for the enlargementof the tube involves the problem of greatly deflecting the straight tubeportions over the entire length thereof. Additionally, the hairpin tubeis likely to rupture at finless portions when enlarged.

[0011] An object of the present invention is to overcome the foregoingproblems and to provide a heat exchanger which is capable of exhibitingthe desired refrigeration performance with the leakage of refrigerantdiminished.

DISCLOSURE OF THE INVENTION

[0012] The present invention provides a finned tube for use in heatexchangers which comprises a hairpin tube having two straight tubeportions, and a plurality of fin groups arranged on the straight tubeportions longitudinally thereof at a spacing, each of the fin groupscomprising a plurality of parallel plate fins extending across and fixedto the two straight tube portions, each of the plate fins having twotube insertion holes spaced apart from each other, the plate fins beingfixedly fitted around an enlarged tube portion of the hairpin tube byinserting the two straight tube portions through the respective holes ofeach plate fin and enlarging the hairpin tube with use of a fluid, thestraight tube portions each having a finless part between each pair ofadjacent fin groups thereon, at least one of all the finless parts ofeach straight tube portion having a restrained small-diameter portionsmaller than the enlarged tube portion in diameter and having apredetermined length.

[0013] The heat exchanger finned tube of the invention comprises ahairpin tube and therefore has no seam, so that the heat exchangerfabricated with use of the finned tube can be diminished in the leakageof refrigerant, consequently permitting the use of a hydrocarbonrefrigerant which is less likely to destroy the ozone layer and exertinfluence on global warming. Since the plate fins are fixedly fittedaround the straight tube portions of the hairpin tube by enlarging thetube with the use of a fluid, inner fins of relatively great height ofprojection are unlikely to collapse, even if formed on the innerperipheral surface of the hairpin tube to afford an increased area ofheat transfer, enabling the heat exchanger to exhibit the desiredrefrigeration performance (heat exchange performance). At least one ofall the finless parts of each straight tube portion of the hairpin tubehas a restrained small-diameter portion of a smaller diameter than theenlarged tube portions and a predetermined length, and thesmall-diameter portion is restrained when the tube is enlarged with theuse of the fluid. Accordingly, the unrestrained portions of the straighttube portion have a relatively short length in the state of the tube tobe enlarged. This prevents the straight tube portion of the hairpin tubefrom being deflected greatly by the enlargement of the tube.

[0014] With the heat exchange finned tube of the invention, therestrained small-diameter portion may be provided in each of the finlessparts of each straight tube portion of the hairpin tube.

[0015] Since the small-diameter portion is restrained when the tube isenlarged with the fluid in this case, this portion to be restrained andincluded in the straight tube portion becomes greater in length than inthe state thereof before enlargement. This reliably precludes thestraight tube portion of the hairpin tube from being deflected greatlyby the enlargement of the tube.

[0016] With the heat exchanger finned tube of the invention, each ofopposite ends of the restrained small-diameter portion may be madeintegral with the enlarged tube portion by a flaring portion formedtherebetween and increasing in diameter toward the enlarged tubeportion.

[0017] Because each end of the restrained small-diameter portion is madeintegral with the enlarged tube portion by the flaring portion formedtherebetween and increasing in diameter toward the enlarged tubeportion, all the plate fins of each fin group can be reliably fixedlyfitted around the enlarged tube portion when the hairpin tube isenlarged.

[0018] With the heat exchanger finned tube of the invention, therestrained small-diameter portion may be an unenlarged tube portion.

[0019] In bending the finned tube for fabricating a heat exchanger, theunenlarged tube portion is to be bent in this case. The unenlarged tubeportion is not worked on in the preceding step, is therefore free fromwork hardening and is consequently amenable to bending work.

[0020] With the heat exchanger finned tube of the invention, the hairpintube may be integrally provided on an inner peripheral surface thereofwith inner fins extending longitudinally thereof and arranged at aspacing circumferentially thereof.

[0021] The heat exchanger to be fabricated with the use of this finnedtube then exhibits improved heat exchange performance.

[0022] With the heat exchanger finned tube of the invention which tubeis finned also internally, the hairpin tube may have high and low twokinds of inner fins alternately arranged circumferentially thereof andprojecting from the inner peripheral surface of the tube to differentheights, the high inner fins being 0.7 to 1.7 mm in height from thesurface of the hairpin tube, the low inner fins being 0.4 to 1.2 mm inheight from the surface.

[0023] The heat exchanger to be fabricated with the use of this finnedtube is then effectively improved in heat exchange performance.

[0024] With the heat exchanger finned tube of the invention which tubeis finned also internally, all the inner fins may be equal in height andare 0.7 to 1.2 mm in height from the inner peripheral surface of thehairpin tube.

[0025] The heat exchanger to be fabricated with the use of this finnedtube is then effectively improved in heat exchange performance.

[0026] With the heat exchanger finned tube of the invention which tubeis finned also internally, the pitch of the inner fins is 0.4 to 1.6 mm.

[0027] With the heat exchanger finned tube of the invention which tubeis finned also internally, the hairpin tube is 6 to 10 mm in outsidediameter and 0.4 to 0.8 mm in the wall thickness of a circumferentialwall thereof.

[0028] The present invention provides a heat exchanger comprising a heatexchanger finned tube according to claim 1 and formed in a zigzag shapein its entirety by bending the straight tube portions of the hairpintube in the same direction at each pair of finless parts located in thesame position with respect to the longitudinal direction of the straighttube portions, each pair of finless parts adjacent to each otherlongitudinally of the straight tube portions being bent in differentdirections.

[0029] The heat exchanger of the invention has the same advantages aspreviously described with reference to the finned tube.

[0030] With the heat exchanger of the invention, each finless part ofeach of the straight tube portions of the hairpin tube may have arestrained small-diameter portion, and the heat exchanger finned tube isbent at the restrained small-diameter portion of each finless part.

[0031] In this case, the small-diameter portion is restrained when thetube is enlarged with the fluid, is enlarged in no way or only slightly,is worked on in no way or slightly, and is diminished in the degree ofwork hardening. This portion can therefore be bent easily.

[0032] With the heat exchanger of the invention, the hairpin tube may beintegrally provided on an inner peripheral surface thereof with innerfins extending longitudinally thereof and arranged at a spacingcircumferentially thereof. The heat exchanger then exhibits outstandingheat exchange performance.

[0033] With the heat exchanger wherein the hairpin tube of the finnedtube is finned also internally, the hairpin tube may have high and lowtwo kinds of inner fins alternately arranged circumferentially thereofand projecting from the inner peripheral surface of the tube todifferent heights, the high inner fins being 0.7 to 1.7 mm in heightfrom the surface of the hairpin tube, the low inner fins being 0.4 to1.2 mm in height from the surface. The heat exchanger then exhibitsfurther improved heat exchange performance.

[0034] With the heat exchanger wherein the hairpin tube of the finnedtube is finned also internally, all the inner fins are equal in heightand are 0.7 to 1.2 mm in height from the inner peripheral surface of thehairpin tube. The heat exchanger then exhibits still improved heatexchange performance.

[0035] With the two heat exchangers described wherein the hairpin tubeof the finned tube is finned also internally, the pitch of the innerfins may be 0.4 to 1.6 mm.

[0036] With the two heat exchangers described wherein the hairpin tubeof the finned tube is finned also internally, the hairpin tube may be 6to 10 mm in outside diameter and 0.4 to 0.8 mm in the wall thickness ofa circumferential wall thereof.

[0037] The present invention provides a refrigerator which is providedwith a refrigeration cycle having a compressor, a condenser and anevaporator, the evaporator being a heat exchanger according to any oneof claims 10 to 18, and wherein a hydrocarbon refrigerant is used as therefrigerant and circulated at a rate of 1 to 9 kg/h.

[0038] The present invention provides a refrigerated showcase which isprovided with a refrigeration cycle having a compressor, a condenser andan evaporator, the evaporator being a heat exchanger according to anyone of claims 10 to 18, and wherein a hydrocarbon refrigerant is used asthe refrigerant and circulated at a rate of 1 to 9 kg/h.

[0039] The present invention provides a process for producing a finnedtube for use in heat exchangers which process comprises preparing ahairpin tube having two straight tube portions, and a multiplicity ofplate fins each having two tube insertion holes spaced apart from eachother, inserting the two straight tube portions through the respectiveholes of each plate fin to arrange the plate fins in parallel into aplurality of fin groups spaced apart on the straight tube portionslongitudinally thereof and provide a finless part between each pair ofadjacent fin groups on each of the straight tube portions, restrainingat least one of all the finless parts of each straight tube portion by arestraining die having a cylindrical restraining portion with a diametersmaller than the inside diameter of the tube insertion holes of theplate fins, and introducing a fluid into the hairpin tube in this stateto enlarge the tube and fixedly fit the plate fins of each fin grouparound an enlarged tube portion of the hairpin tube.

[0040] The process of the invention is adapted to produce the heatexchanger finned tube having the foregoing advantages relatively easily.The hairpin tube is enlarged with a fluid introduced thereinto, with atleast one of the finless parts of each straight tube portion restrainedby a die having a cylindrical restraining portion with a diametersmaller than the inside diameter of the tube insertion holes of theplate fins, with the result that the straight tube portion of thehairpin tube is precluded from being deflected greatly by the tubeenlarging operation.

[0041] In the process of the invention for producing a heat exchangerfinned tube, each of the finless parts of each straight tube portion ofthe hairpin tube may be restrained by the restraining die. This reliablyprecludes the straight tube portion of the hairpin tube from beingdeflected greatly by the enlargement of the tube. Moreover, the hairpintube is prevented from being ruptured by the enlargement at the portionsthereof having no fin group.

[0042] With the process of the invention for producing a heat exchangerfinned tube, the restraining die may have a cavity comprising acylindrical restraining portion having a predetermined length, twoflaring portions extending from respective opposite ends of therestraining portion and increasing in diameter outward longitudinally ofthe restraining portion, and tube enlargement permitting portionsextending from respective larger ends of the flaring portions and havingan inside diameter not smaller than the inside diameter of the holes ofthe plate fins.

[0043] With the process of the invention for producing a heat exchangerfinned tube, the restraining portion may have an inside diameter equalto the outside diameter of the hairpin tube before enlargement. Therestrained small-diameter portion present in the finless part of thefinned tube obtained in this case is an unenlarged tube portion. Inbending the finned tube for fabricating a heat exchanger, the unenlargedtube portion is to be bent. The unenlarged tube portion is not worked onin the preceding step, is therefore free from work hardening and canconsequently be bent easily.

[0044] In the process of the invention for producing a heat exchangefinned tube, the hairpin tube may be integrally provided on an innerperipheral surface thereof with inner fins extending longitudinallythereof and arranged at a spacing circumferentially thereof.

[0045] The plate fins of each fin group are fixedly fitted around thestraight tube portion of the hairpin tube by enlarging the hairpin tubewith the fluid introduced thereinto, so that the inner fins areprevented from collapsing by the enlargement of the tube. The heatexchanger fabricated with the use of the finned tube therefore exhibitsoutstanding heat exchange performance.

[0046] In the process for producing a heat exchange finned tube which isfinned also internally, the hairpin tube may have high and low two kindsof inner fins alternately arranged circumferentially thereof andprojecting from the inner peripheral surface of the tube to differentheights, the high inner fins being 0.7 to 1.7 mm in height from thesurface of the hairpin tube, the low inner fins being 0.4 to 1.2 mm inheight from the surface.

[0047] In the process for producing a heat exchange finned tube which isfinned also internally, all the inner fins may be equal in height andare 0.7 to 1.2 mm in height from the inner peripheral surface of thehairpin tube.

[0048] In the process for producing the two heat exchange finned tubeswhich are finned also internally, the pitch of the inner fins is 0.4 to1.6 mm.

[0049] In the process for producing the two heat exchange finned tubeswhich are finned also internally, the hairpin tube is 6 to 10 mm inoutside diameter and 0.4 to 0.8 mm in the wall thickness of acircumferential wall thereof.

[0050] The present invention provides a process for fabricating a heatexchanger comprising a heat exchanger finned tube produced by a processaccording to claim 21, the heat exchanger being formed in a zigzag shapein its entirety by bending the straight tube portions of the hairpintube in the same direction at each pair of finless parts located in thesame position with respect to the longitudinal direction of the straighttube portions, each pair of finless parts adjacent to each otherlongitudinally of the straight tube portions being bent in differentdirections.

[0051] This process for fabricating the heat exchanger also has the sameadvantages as already described with reference to the finned tube.

[0052] In the process of the invention for fabricating a heat exchangerby using a heat exchanger finned tube produced by the process accordingto claim 22, the finned tube may be bent at the portion of the finlesspart restrained by the restraining portion of the restraining die.

[0053] In this case, the restrained portion is bent when the finned tubeis to be bent zigzag. The restrained portion is enlarged in no way oronly slightly, is therefore worked on in no way or slightly, and isdiminished in the degree of work hardening. This portion can thereforebe bent easily.

[0054] In the process of the invention for fabricating a heat exchanger,the hairpin tube of the heat exchanger finned tube may be integrallyprovided on an inner peripheral surface thereof with inner finsextending longitudinally thereof and arranged at a spacingcircumferentially thereof.

[0055] In the process for fabricating a heat exchanger wherein thehairpin tube is finned internally, the hairpin tube may have high andlow two kinds of inner fins alternately arranged circumferentiallythereof and projecting from the inner peripheral surface of the tube todifferent heights, the high inner fins being 0.7 to 1.7 mm in heightfrom the surface of the hairpin tube, the low inner fins being 0.4 to1.2 mm in height from the surface.

[0056] In the process for fabricating a heat exchanger wherein thehairpin tube is finned internally, all the inner fins are equal inheight and are 0.7 to 1.2 mm in height from the inner peripheral surfaceof the hairpin tube.

[0057] In the process for fabricating the two heat exchangers whereinthe hairpin tube is finned internally, the pitch of the inner fins is0.4 to 1.6 mm.

[0058] In the process for fabricating the two heat exchangers whereinthe hairpin tube is finned internally, the hairpin tube is 6 to 10 mm inoutside diameter and 0.4 to 0.8 mm in the wall thickness of acircumferential wall thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0059]FIG. 1 is a plan view partly omitted and showing a finned tube ofthe invention for use in heat exchangers.

[0060]FIG. 2 is an enlarged view in section taken along the line II-IIin FIG. 1.

[0061]FIG. 3 is a sectional view showing a process for producing thefinned tube of FIG. 1, a hairpin tube being shown before enlargement.

[0062]FIG. 4 shows a process for producing the heat exchanger finnedtube of FIG. 1, (a) being an enlarged fragmentary view in section of thehairpin tube before enlargement; (b) being an enlarged fragmentary viewin section of the hairpin tube after enlargement.

[0063]FIG. 5 is a fragmentary perspective view showing a process forfabricating a heat exchanger using the finned tube.

[0064]FIG. 6 is a perspective view showing the overall construction ofthe heat exchanger of the invention.

[0065]FIG. 7 is a sectional view corresponding to FIG. 2 and showinganother embodiment of finned tube for use in heat exchangers.

[0066]FIG. 8 is a sectional view corresponding to FIG. 2 and showing aheat exchanger finned tube for use in a comparative device 1.

[0067]FIG. 9 is a graph showing the results of performance test inExperimental Example 1.

BEST MODE OF CARRYING OUT THE INVENTION

[0068] Embodiments of the present invention will be described below withreference to the drawings. The term “aluminum” as used in the followingdescription includes aluminum alloys in addition to pure aluminum.Further in the following description, the left- and right-hand sides ofFIGS. 1 to 4 will be referred to as “left” and “right”, respectively.

[0069]FIGS. 1 and 2 show a finned tube for use in heat exchanger, FIGS.3 and 4 show a process for producing the finned tube, and FIG. 5 shows aprocess for fabricating a heat exchanger with the use of the finnedtube. Further FIG. 6 shows the overall construction of the heatexchanger fabricated using the finned tube.

[0070] With reference to FIGS. 1 and 2, a finned tube 10 for use in heatexchangers comprises a hairpin tube 11 of aluminum, and a plurality offin groups 13 arranged on two straight tube portions 11 a of the hairpintube 11 longitudinally thereof at a spacing. The straight tube portions11 a each have a finless part 19 between each pair of adjacent fingroups 13 thereon. The fin group 13 comprises a plurality of parallelaluminum plate fins 12 extending across and fixed to the two straighttube portions 11 a of the hairpin tube 11.

[0071] The hairpin tube 11 is integrally provided with high and low twokinds of inner fins 30, 31 projecting from the inner peripheral surfaceof the tube to different heights, extending longitudinally thereof, andalternately arranged circumferentially thereof at a spacing. The innerfins 30, 31 project toward the center of the hairpin tube 11. The highinner fins 30 are 0.7 to 1.7 mm in height h1 as measured from the innerperipheral surface of the hairpin tube 11, and the low inner fins 31 are0.4 to 1.2 mm in height h2 as measured from the surface of the hairpintube 11. The pitch p of the inner fins 30, 31 is 0.4 to 1.6 mm. Thepitch p of the inner fins 30, 31 is the circumferential distance, asmeasured in cross section on the outer periphery of the hairpin tube 11,between two straight lines connecting the center line of the hairpintube 11 and the centers of the thicknesses of a pair of adjacent innerfins 30, 31. The hairpin tube 11 is 6 to 10 mm in outside diameter, and0.4 to 0.8 mm in the thickness of the circumferential wall thereof.

[0072] Each of the plate fins 12 has two tube insertion holes 12 a. Theplate fins 12 are fixedly fitted around the straight tube portions 11 aof the hairpin tube 11 by inserting the two straight tube portions 11 athrough the respective two holes 12 a of each plate fin 12 and enlargingthe hairpin tube at the portions thereof where the fins groups 13 are tobe provided, with use of a fluid such as water, oil or air. The enlargedtube portions are indicated at 14. The hairpin tube 11 has a bentportion 11 b which is enlarged in its entirety and given the samediameter as the enlarged portions 14 for fixing to the straight tubeportions 11 a the group 13 of the plate fins 12 adjacent to the bentportion 11 b. Each enlarged tube portion 14 has a larger length than thewidth of the fin group 13 in the leftward or rightward direction, andhas left and right ends positioned leftwardly and rightwardly externallyof the respective plate fins 12 at the left and right ends of the fingroup 13.

[0073] Each of the finless parts 19 has a restrained small-diameterportion 15 of a predetermined length. Each of left and right ends of therestrained small-diameter portion 15 is made integral with the enlargedtube portion 14 by a flaring portion 16 formed therebetween andincreasing in diameter toward the enlarged tube portion 14. Therestrained small-diameter portions 15 of the two straight tube portions11 a are in the same position with respect to the longitudinal directionof the tube portions 11 a. The hairpin tube 11 has portions alsoproviding restrained small-diameter portions 17 and closer to theopenings thereof than the enlarged tube portions 14 for fixing the platefins 12 of the fin group 13 at the open ends (left ends) of the hairpintube 11. These small-diameter portions 17 each have a right end madeintegral with the enlarged tube portion 14 by a flaring portion 18formed therebetween and increasing in diameter toward the enlarged tubeportion 14. The restrained small-diameter portions 15, 17 are each inthe form of an unenlarged tube portion which is not enlarged when thestraight tube portions are enlarged as described above. Instead of beingunenlarged tube portions, the restrained small-diameter portions 15, 17may be slightly enlarged tube portions which have a diameter smallerthan the inside diameter of the tube insertion holes 12 a of the platefin 12, i.e., the outside diameter of the enlarged tube portions 14, butwhich are slightly enlarged.

[0074] The finned tube 10 for use in heat exchangers is produced in themanner shown in FIGS. 3 and 4.

[0075] A hairpin tube 11 of aluminum having two straight tube portions,and a multiplicity of aluminum plate fins 12 are prepared. Each of theplate fins 12 has two tube insertion holes 12 a spaced apart from eachother. The two straight tube portions 11 a of the hairpin tube 11 areinserted through the respective holes 12 a of each plate fin 12 toarrange the plate fins 12 in parallel-into a plurality of fin groups 13spaced apart on the straight tube portions 11 a longitudinally thereof.A restraining die 20 is then used for restraining the finless part 19between each pair of adjacent fin groups 13 on the straight tubeportions 11 a of the hairpin tube 11. The restraining die 20 comprisestwo die members 20 a, 20 a, and has a cavity 24 comprising a cylindricalrestraining portion 21 having a predetermined length and an insidediameter equal to the outside diameter of the hairpin tube 11 beforeenlargement, two flaring portions 22 extending from respective oppositeends of the restraining portion 21 and increasing in diameter outwardlongitudinally of the restraining portion, and short cylindrical tubeenlargement permitting portions 23 extending from respective larger endsof the flaring portions 22 and having an inside diameter not smallerthan the inside diameter of the holes 12 a of the plate fins 12 [seeFIG. 4(a)].

[0076] Further the bent portion 11 b of the hairpin tube 11 isrestrained by a second restraining die 25. This die 25 has a U-shapedcavity 26 circular in cross section. The cavity 26 has an insidediameter larger than the outside diameter of the hairpin tube 11 beforeenlargement and equal to the inside diameter of the tube enlargementpermitting portion 23 of the first restraining die 20. The opposite endportions of the hairpin tube 11 are restrained by a fluid introductionjig 27. The jig 27 has two cylindrical restraining portions 28 having aninside diameter equal to the outside diameter of the hairpin tube 11before enlargement, and two fluid inlet passageways 29 communicatingwith the respective restraining portions 28 (see FIG. 3). The jig 27 hasa flaring portion 28b extending from the right end of each restrainingportion 28 and increasing in diameter rightward, and an enlargementpermitting portions 28 b extending from the larger end of the flaringportion 28 a and having the same inside diameter as the enlargementpermitting portion 23 of the first die 20.

[0077] Subsequently, a pressure fluid, such as water, oil or air, isintroduced from the inlet passageways 29 of the jig 27 into the hairpintube 11 in this state to enlarge the hairpin tube 11 at the portionsthereof except the portions restrained by the restraining portions 21 ofthe die 20 and the restraining portions 28 of the jig 27 and to fixedlyfit the plate fins 12 of the fin groups 13 around the enlarged tubeportions 14 formed in the straight tube portions 11 a of the hairpintube 11. The restrained small-diameter portions 15, 17 and flaringportions 16, 18 are formed by this enlarging operation [see FIG. 4(b)].In this way, the heat exchanger finned tube 10 is produced.

[0078] The use of the restraining dies 20 described above in the processprevents the straight portions 11 a of the hairpin tube 11 from beingdeflected greatly with the pressure fluid introduced into the hairpintube 11, further precluding the restrained small-diameter portions 15from wrinkling in the circumferential wall and the straight tubeportions 11 a from deforming longitudinally thereof due to wrinklingwhen the pressure fluid is introduced into the hairpin tube 11. Sincethe tube is enlarged with the pressure fluid, the inner fins 30, 31 areprevented from collapsing. The finless parts 19 of the hairpin tube 11are also prevented from rupturing.

[0079] As shown in FIG. 5, the finned tube 10 is bent at the restrainedsmall-diameter portions 15 between the adjacent fin groups 13, wherebythe tube is formed zigzag in its entirety. FIG. 6 shows a heat exchanger1 thus fabricated for use as an evaporator in refrigerators orrefrigerated showcases. Stated more specifically, the straight tubeportions 11 a of the hairpin tube 11 are bent in the same direction atthe restrained small-diameter portions 15 of each pair of finless parts19 which are located in the same position with respect to thelongitudinal direction of the straight tube portions 11 a so that astraight line through the lengthwise centers of the portions 15 will bethe center of the curvature, and each pair of finless parts 19 adjacentto each other longitudinally of the straight tube portions 11 a are bentin different directions, whereby the hairpin tube 11 is bent zigzag inits entirety.

[0080] Thus, the finned tube 10 is bent at the restrained small-diameterportions 15 and can therefore be bent easily. Because the restrainedsmall-diameter portions 15 are not worked on in the preceding steps,these portions 15 are free of work hardening and can consequently bebent easily.

[0081] With reference to FIG. 6, the heat exchanger 1 comprises a zigzagheat exchange tube 2 comprising a hairpin tube 11 bent zigzag, and fingroups 13 provided around each straight tube portion 2 a of the zigzagheat exchange tube 2 and each comprising a plurality of parallel platefins 12. A plurality of bent portions 2 b at the left and right sides ofthe zigzag heat exchange tube 2 each comprise a restrainedsmall-diameter portion 15. Although not shown, the bent portions 2 b ofthe tube 2 at the left and right are held by respective side plates.

[0082] The heat exchanger 1 is used as the evaporator of a refrigeratorwhich is provided with a refrigeration cycle having a compressor,condenser and evaporator, and wherein a hydrocarbon refrigerant is usedas the refrigerant. In this refrigerator, the refrigerant is circulatedat a low rate of 1 to 9 kg/h.

[0083] The heat exchanger 1 is used also as the evaporator of arefrigerated showcase which is provided with a refrigeration cyclehaving a compressor, condenser and evaporator, and wherein a hydrocarbonrefrigerant is used as the refrigerant. In this refrigerated showcase,the refrigerant is circulated at a low rate of 1 to 9 kg/h.

[0084]FIG. 7 shows a modified heat exchanger finned tube.

[0085] With reference to FIG. 7, a hairpin tube 11 is integrallyprovided with a plurality of inner fins 32 projecting from the innerperipheral surface of the tube to equal heights, extendinglongitudinally thereof, and arranged circumferentially thereof at aspacing. The inner fins 32 are 0.7 to 1.2 mm in height h3 as measuredfrom the inner peripheral surface of the hairpin tube 11. The inner fins32 have the same pitch p as those already described. The hairpin tube 11is 6 to 10 mm in outside diameter, and 0.4 to 0.8 mm in the thickness ofthe circumferential wall thereof.

[0086] Experimental examples are given below wherein heat exchangers ofthe invention and comparative heat exchangers were used.

EXPERIMENTAL EXAMPLE 1

[0087] Prepared were a heat exchanger 1 (invention device 1) comprisinga hairpin tube 11 having the cross section shown in FIG. 2, and a heatexchanger 1 (invention device 2) comprising a hairpin tube 11 having thecross section shown in FIG. 7. The hairpin tube 11 of the heat exchanger1 as the invention device 1 was 8 mm in outside diameter, 0.61 mm incircumferential wall thickness, 1.2 mm in the height h1 of projection ofhigh inner fins 30, 0.65 mm in the height h2 of projection of low innerfins 31, and 30 in the combined number of two kinds of inner fins 30,31. The hairpin tube 11 of the heat exchanger 1 as the invention device2 was 8 mm in outside diameter, 0.61 mm in circumferential wallthickness, 1.2 mm in the height h3 of projection of inner fins 32, and30 in the number of inner fins 32.

[0088] Also prepared was a heat exchanger (comparative device 1) havingthe same construction as the invention device 1 except that the hairpintube 40 used had the cross section shown in FIG. 8. The hairpin tube 40was integrally provided on its inner peripheral surface with a pluralityof inner fins 41 extending longitudinally of the tube and arranged at aspacing circumferentially thereof. The hairpin tube 40 was 8 mm inoutside diameter, 0.61 mm in circumferential wall thickness and 30 inthe number of inner fins 41. The comparative device 1 was fabricated bythe conventional second process previously described, and the inner fins41 were 0.65 mm in the height of projection before the enlargement ofthe tube. However, fins were somewhat collapsed at their inner ends bythe enlarging mandrel.

[0089] Evaporators comprising the invention devices 1, 2 and comparativedevice 1, respectively, were used and checked for performance at aninlet temperature of −19 to −22° C., refrigerant evaporation temperatureof −30° C., degree of superheat of 3° C., refrigerant pressure, upstreamfrom expansion valve, of 1.06 MPa and refrigerant circulation rate of 2to 4 kg/h. The results obtained are given in FIG. 9, which reveals thatthe invention devices 1 and 2 are approximately 10% higher inperformance.

EXPERIMENTAL EXAMPLE 2

[0090] Evaporators comprising the invention devices 1, 2 and comparativedevice 1 were incorporated into refrigerators, which were installed inan atmosphere having a temperature of 25° C. and relative humidity of70%. The refrigerators were then tested for power consumption byoperating the compressor intermittently by on/off control with the doorclosed. As a result, the refrigerator incorporating the invention device1 as its evaporator was found to be 2% lower in power consumption thanthe refrigerator incorporating the comparative device 1 as theevaporator. Similarly, the refrigerator incorporating the inventiondevice 2 as its evaporator was 1.3% lower in power consumption than therefrigerator incorporating the comparative device I as the evaporator.

[0091] Industrial Applicability

[0092] The heat exchanger finned tube of the invention is used forfabricating heat exchangers for use as evaporators in refrigerationdevices such as refrigerators and refrigerated showcases, and is suitedespecially for fabricating heat exchangers useful as the evaporators ofrefrigeration devices wherein hydrocarbon refrigerant is used.

1. A finned tube for use in heat exchangers which comprises a hairpintube having two straight tube portions, and a plurality of fin groupsarranged on the straight tube portions longitudinally thereof at aspacing, each of the fin groups comprising a plurality of parallel platefins extending across and fixed to the two straight tube portions, eachof the plate fins having two tube insertion holes spaced apart from eachother, the plate fins being fixedly fitted around an enlarged tubeportion of the hairpin tube by inserting the two straight tube portionsthrough the respective holes of each plate fin and enlarging the hairpintube with use of a fluid, the straight tube portions each having afinless part between each pair of adjacent fin groups thereon, at leastone of all the finless parts of each straight tube portion having arestrained small-diameter portion smaller than the enlarged tube portionin diameter and having a predetermined length.
 2. A finned tube for usein heat exchangers according to claim 1 wherein the restrainedsmall-diameter portion is provided in each of the finless parts of eachstraight tube portion of the hairpin tube.
 3. A finned tube for use inheat exchangers according to claim 1 wherein each of opposite ends ofthe restrained small-diameter portion is made integral with the enlargedtube portion by a flaring portion formed therebetween and increasing indiameter toward the enlarged tube portion.
 4. A finned tube for use inheat exchangers according to claim 1 wherein the restrainedsmall-diameter portion is an unenlarged tube portion.
 5. A finned tubefor use in heat exchangers according to claim 1 wherein the hairpin tubeis integrally provided on an inner peripheral surface thereof with innerfins extending longitudinally thereof and arranged at a spacingcircumferentially thereof.
 6. A finned tube for use in heat exchangersaccording to claim 5 wherein the hairpin tube has high and low two kindsof inner fins alternately arranged circumferentially thereof andprojecting from the inner peripheral surface of the tube to differentheights, the high inner fins being 0.7 to 1.7 mm in height from thesurface of the hairpin tube, the low inner fins being 0.4 to 1.2 mm inheight from the surface.
 7. A finned tube for use in heat exchangersaccording to claim 5 wherein all the inner fins are equal in height andare 0.7 to 1.2 mm in height from the inner peripheral surface of thehairpin tube.
 8. A finned tube for use in heat exchangers according toclaim 6 wherein the pitch of the inner fins is 0.4 to 1.6 mm.
 9. Afinned tube for use in heat exchangers according to claim 6 wherein thehairpin tube is 6 to 10 mm in outside diameter and 0.4 to 0.8 mm in thewall thickness of a circumferential wall thereof.
 10. A heat exchangercomprising a heat exchanger finned tube according to claim 1 and formedin a zigzag shape in its entirety by bending the straight tube portionsof the hairpin tube in the same direction at each pair of finless partslocated in the same position with respect to the longitudinal directionof the straight tube portions, each pair of finless parts adjacent toeach other longitudinally of the straight tube portions being bent indifferent directions.
 11. A heat exchanger according to claim 10 whereineach finless part of each of the straight tube portions of the hairpintube has a restrained small-diameter portion, and the heat exchangerfinned tube is bent at the restrained small-diameter portion of eachfinless part.
 12. A heat exchanger according to claim 10 wherein thehairpin tube is integrally provided on an inner peripheral surfacethereof with inner fins extending longitudinally thereof and arranged ata spacing circumferentially thereof.
 13. A heat exchanger according toclaim 12 wherein the hairpin tube has high and low two kinds of innerfins alternately arranged circumferentially thereof and projecting fromthe inner peripheral surface of the tube to different heights, the highinner fins being 0.7 to 1.7 mm in height from the surface of the hairpintube, the low inner fins being 0.4 to 1.2 mm in height from the surface.14. A heat exchanger according to claim 13 wherein the pitch of theinner fins is 0.4 to 1.6 mm.
 15. A heat exchanger according to claim 13wherein the hairpin tube is 6 to 10 mm in outside diameter and 0.4 to0.8 mm in the wall thickness of a circumferential wall thereof.
 16. Aheat exchanger according to claim 12 wherein all the inner fins areequal in height and are 0.7 to 1.2 mm in height from the innerperipheral surface of the hairpin tube.
 17. A heat exchanger accordingto claim 16 wherein the pitch of the inner fins is 0.4 to 1.6 mm.
 18. Aheat exchanger according to claim 16 wherein the hairpin tube is 6 to 10mm in outside diameter and 0.4 to 0.8 mm in the wall thickness of acircumferential wall thereof.
 19. A refrigerator which is provided witha refrigeration cycle having a compressor, a condenser and anevaporator, the evaporator being a heat exchanger according to claim 10,and wherein a hydrocarbon refrigerant is used as the refrigerant andcirculated at a rate of 1 to 9 kg/h.
 20. A refrigerated showcase whichis provided with a refrigeration cycle having a compressor, a condenserand an evaporator, the evaporator being a heat exchanger according toclaim 10, and wherein a hydrocarbon refrigerant is used as therefrigerant and circulated at a rate of 1 to 9 kg/h.
 21. A process forproducing a finned tube for use in heat exchangers which processcomprises preparing a hairpin tube having two straight tube portions,and a multiplicity of plate fins each having two tube insertion holesspaced apart from each other, inserting the two straight tube portionsthrough the respective holes of each plate fin to arrange the plate finsin parallel into a plurality of fin groups spaced apart on the straighttube portions longitudinally thereof and provide a finless part betweeneach pair of adjacent fin groups on each of the straight tube portions,restraining at least one of all the finless parts of each straight tubeportion by a restraining die having a cylindrical restraining portionwith a diameter smaller than the inside diameter of the tube insertionholes of the plate fins, and introducing a fluid into the hairpin tubein this state to enlarge the tube and fixedly fit the plate fins of eachfin group around an enlarged tube portion of the hairpin tube.
 22. Aprocess for producing a finned tube for use in heat exchangers accordingto claim 21 wherein each of the finless parts of each straight tubeportion of the hairpin tube is restrained by the restraining die.
 23. Aprocess for producing a finned tube for use in heat exchangers accordingto claim 21 wherein the restraining die has a cavity comprising acylindrical restraining portion having a predetermined length, twoflaring portions extending from respective opposite ends of therestraining portion and increasing in diameter outward longitudinally ofthe restraining portion, and tube enlargement permitting portionsextending from respective larger ends of the flaring portions and havingan inside diameter not smaller than the inside diameter of the holes ofthe plate fins.
 24. A process for producing a finned tube for use inheat exchangers according to claim 21 wherein the restraining portionhas an inside diameter equal to the outside diameter of the hairpin tubebefore enlargement.
 25. A process for producing a finned tube for use inheat exchangers according to claim 21 wherein the hairpin tube isintegrally provided on an inner peripheral surface thereof with innerfins extending longitudinally thereof and arranged at a spacingcircumferentially thereof.
 26. A process for producing a finned tube foruse in heat exchangers according to claim 25 wherein the hairpin tubehas high and low two kinds of inner fins alternately arrangedcircumferentially thereof and projecting from the inner peripheralsurface of the tube to different heights, the high inner fins being 0.7to 1.7 mm in height from the surface of the hairpin tube, the low innerfins being 0.4 to 1.2 mm in height from the surface.
 27. A process forproducing a finned tube for use in heat exchangers according to claim 25wherein all the inner fins are equal in height and are 0.7 to 1.2 mm inheight from the inner peripheral surface of the hairpin tube.
 28. Aprocess for producing a finned tube for use in heat exchangers accordingto claim 26 wherein the pitch of the inner fins is 0.4 to 1.6 mm.
 29. Aprocess for producing a finned tube for use in heat exchangers accordingto claim 26 wherein the hairpin tube is 6 to 10 mm in outside diameterand 0.4 to 0.8 mm in the wall thickness of a circumferential wallthereof.
 30. A process for fabricating a heat exchanger comprising aheat exchanger finned tube produced by a process according to claim 21,the heat exchanger being formed in a zigzag shape in its entirety bybending the straight tube portions of the hairpin tube in the samedirection at each pair of finless parts located in the same positionwith respect to the longitudinal direction of the straight tubeportions, each pair of finless parts adjacent to each otherlongitudinally of the straight tube portions being bent in differentdirections.
 31. A process for fabricating a heat exchanger according toclaim 30 by using a heat exchanger finned tube, wherein the finned tubeis bent at the portion of the finless part restrained by the restrainingportion of the restraining die.
 32. A process for fabricating a heatexchanger according to claim 30 wherein the hairpin tube of the heatexchanger finned tube is integrally provided on an inner peripheralsurface thereof with inner fins extending longitudinally thereof andarranged at a spacing circumferentially thereof.
 33. A process forfabricating a heat exchanger according to claim 32 wherein the hairpintube has high and low two kinds of inner fins alternately arrangedcircumferentially thereof and projecting from the inner peripheralsurface of the tube to different heights, the high inner fins being 0.7to 1.7 mm in height from the surface of the hairpin tube, the low innerfins being 0.4 to 1.2 mm in height from the surface.
 34. A process forfabricating a heat exchanger according to claim 32 wherein all the innerfins are equal in height and are 0.7 to 1.2 mm in height from the innerperipheral surface of the hairpin tube.
 35. A process for fabricating aheat exchanger according to claim 33 wherein the pitch of the inner finsis 0.4 to 1.6 mm.
 36. A process for fabricating a heat exchangeraccording to claim 33 wherein the hairpin tube is 6 to 10 mm in outsidediameter and 0.4 to 0.8 mm in the wall thickness of a circumferentialwall thereof.